A sinle injection of either 40, 200 or 500 micrograms of 5 alpha-dihydrotestosterone (5 alpha-DHT) into the proximal corpus epididymis (PCE) after ligation of the ductuli efferentes and the corpus epididymis did not alter the fertilizing capacty of the PCE sperm as compared to the control when tested by artificial insemination (A.I.) 12 days after treatment (20% in the control vs. 5-26% in 5 alpha-DHT treated). Out of three 5 alpha-reductase inhibitors tested, Colprone (6,17-dimethylpregna-4, 6-diene-3,20-dione) at a dose level of 200 micrograms/PCE for 4-8 days or 17 beta C (3-oxo-4-androstene-17 beta-carboxylic acid) at a dose level of 200 micrograms/PCE for 12 days induced a slight increase in fertilizing capacity of the PCE sperm as judged by the percent of females showing fertilized eggs and the proportion of eggs fertilized after A.I.; the third inhibitor, SKF #7690 Polymorph A was apparently ineffective to alter the fertilizing capacity of the PCE sperm. In order to study the effects of different compounds on the fertilizing capacity of the cauda epididymal sperm, the vas deferens and the corpus epididymis were ligated and a tested compound was injected into the cauda epididymis. It was observed that a dose as high as 500 micrograms/cauda epididymis of 5 alpha-DHT caused no marked changes in the fertilizing capacity of the cauda epididymal sperm as compared to the control when tested 12 days later. However, 17 beta C at a dose of 200 micrograms/cauda epididymis and tested 12 days later by A.I. indicated a significant inhibition (89% in control vs. 8.3% in 17 beta C treated). More studies are underway to substantiate these observations.